Add test coverage thresholds.

jest.config.js

@@ -25,4 +25,14 @@ module.exports = {
   // It is reflected in properties such as location.href.
   // @see: https://github.com/facebook/jest/issues/6769
   testURL: 'http://localhost/',
+
+  // @see: https://jestjs.io/docs/en/configuration#coveragethreshold-object
+  coverageThreshold: {
+    global: {
+      statements: 100,
+      branches: 95,
+      functions: 100,
+      lines: 100,
+    },
+  },
 };